Re: [PATCH-block v3 1/2] bdi, blk-cgroup: Fix potential UAF of blkcg

[Jens Axboe <axboe@kernel.dk>]

On 12/13/22 12:53 PM, Waiman Long wrote:
> 
> On 12/13/22 14:29, Tejun Heo wrote:
>> On Tue, Dec 13, 2022 at 01:44:45PM -0500, Waiman Long wrote:
>>> Commit 59b57717fff8 ("blkcg: delay blkg destruction until after
>>> writeback has finished") delayed call to blkcg_destroy_blkgs() to
>>> cgwb_release_workfn(). However, it is done after a css_put() of blkcg
>>> which may be the final put that causes the blkcg to be freed as RCU
>>> read lock isn't held.
>>>
>>> Another place where blkcg_destroy_blkgs() can be called indirectly via
>>> blkcg_unpin_online() is from the offline_css() function called from
>>> css_killed_work_fn(). Over there, the potentially final css_put() call
>>> is issued after offline_css().
>>>
>>> By adding a css_tryget() into blkcg_destroy_blkgs() and warning its
>>> failure, the following stack trace was produced in a test system on
>>> bootup.
>> This doesn't agree with the code anymore. Otherwise
>>
>> Acked-by: Tejun Heo <tj@kernel.org>
> 
> Sorry, I overlooked the commit log in my update. I will update it if I need another version, or Jens can make the following edit:
> 
> css_tryget() -> percpu_ref_is_zero().

Since the other one also needs an edit, would be great if you could
just send out a v4.

-- 
Jens Axboe